This invention relates to a process for producing sulfuric acid from gases containing SO3 and gaseous nitrosyl sulfuric acid by contacting with highly concentrated sulfuric acid.
Processes for producing sulfuric acid are known. In Rompps Chemie-Lexikon, 8th edition, 1987, pages 3760 to 3764, there is described a process for producing sulfuric acid. As raw material sulfur dioxide is used, which is contacted with a catalyst and catalytically converted to SO3. The SO3 discharged from the catalytic furnace is subsequently introduced into a 98% sulfuric acid, where first of all disulfuric acid (H2S2O7) is formed, which by adding water is converted to sulfuric acid. In this way, particularly highly concentrated sulfuric acids as well as various types of oleum can be obtained. This process, however, has the disadvantage that in the catalytic conversion of SO2 to SO3, in dependence on the content of nitrogen oxide after the SO2 production, there is also formed nitrosyl sulfuric acid, which likewise gets into the highly concentrated sulfuric acid and must be removed with a relatively great technical effort. In the DE-OS 17 92 348 there is described a process for producing sulfuric acid free from ammonia and nitrose, where it is provided to add hydrazine compounds to the sulfuric acid for the purpose of denitration. However, this is a relatively time- and cost-consuming process. The DE-195 16 303 describes a process for reducing the NOx content of sulfuric acid, where NOx is reduced with SO2 to form N2, and N2 is discharged, so that SO2 is used for denitration. This process has the disadvantage that the nitrosyl sulfuric acid obtained in the absorption towers cannot be removed due to the high concentration of sulfuric acid.
It is the object underlying the invention to create a process for producing sulfuric acid from gases containing SO3 and gaseous nitrosyl sulfuric acid by contacting with highly concentrated sulfuric acid, where there is no enrichment of nitrosyl sulfuric acid in the absorption towers.
The object underlying the invention is solved in that before contacting with highly concentrated sulfuric acid the gases are passed through at least one cooler and are cooled to 160 to 130xc2x0 C. xe2x80x9cNitrosyl sulfuric acidxe2x80x9d is understood to be the gaseous nitrosyl hydrogensulfate (NO+HSO4xe2x88x92 or HO3SONO). The concentration of the highly concentrated sulfuric acid generally lies in the range between 94 and 98%. As coolers, conventional gas coolers or heat exchangers may be used, such as shell-and-tube heat exchangers or ribbed tube-coil heat exchangers. It was surprisingly found out that the gases containing SO3 and gaseous nitrosyl sulfuric acid can advantageously be cooled in at least one cooler, so that the nitrosyl sulfuric acid condensates out of the gases and does therefore no longer come in contact with the highly concentrated sulfuric acid in the absorption tower. The condensate containing nitrosyl sulfuric acid can advantageously be denitrated by means of the process described in DE-195 16 303.
A preferred embodiment of the invention consists in that the gases are cooled to 160 to 150xc2x0 C. Advantageously, the nitrosyl sulfuric acid condensates out almost exclusively in the cooler. Since the gases containing SO3 and gaseous nitrosyl sulfuric acid generally also contain a certain amount of steam, the gases will generally also have a content of gaseous sulfuric acid, which at lower cooler temperatures would condensate out together with the nitrosyl sulfuric acid, which is, however, not desired. But when the gases are cooled to 160 to 150xc2x0 C., it is ensured that the gaseous sulfuric acid will not condensate out, but can subsequently be introduced into the absorption tower.
In accordance with a further preferred aspect of the invention it is provided that the gases are catalytically oxidized after contacting with highly concentrated sulfuric acid for the enrichment of SO3, are subsequently again passed through at least one cooler and are cooled to 160 to 130xc2x0 C., and thereafter are again contacted with highly concentrated sulfuric acid. The advantage is that the gases which are removed from the absorption tower and contain still larger amounts of SO2 can again be used for the production of sulfuric acid. Furthermore, the concentration of SO2 in the pure gas is reduced.
The invention will subsequently be explained in detail and by way of example with reference to the drawing (FIG. 1 and 2), wherein: